Bimetallic snap-action switch



Jan. 24, 1956 J. HUBER 2,732,461

BIMETALLIC SNAP-ACTION swnca Filed April 9, 1952 INVENTOR Xd/M/Z M, BYQMJM" QZA/QMG ATTORNEYS United States Patent BIMETALLIC sNAP-AcrIoN SWITCH Jakob Huber, Baden, Switzerland, assignor to J. Huber & Cie. Aktiengesellschaft, Baden, Switzerland, a corporation of Switzerland Application April 9, 1952, Serial No. 281,358 Claims priority, application Switzerland April 10, 1951 9 Claims. (Cl. 200133) This invention relates to bimetallic snap-action switches such as employed for the supervision of electrical machines and other apparatus, and more particularly to switches which may be manufactured in ex tremely small sizes to respond to temperature changes at a critical point or zone of the controlled machine or apparatus.

Objects of the invention are to provide bimetallic snapaction switches in which domed bimetallic disks are housed in lenticular casings to act as switch blades coopcrating with relatively stationary contacts supported on and axially of one or both of the lenticular walls of the casing. Other objects are to provide switches of the type stated in which the lenticular casing is formed of two plates with dished rims which meet at substantially the median plane of the casing and flare away from each other to provide two opposed seats against which the edge portion of the bimetallic disks rests in alternation according to the ambient temperature, the edge portion of the disk being confined between but not otherwise mechanically connected to the casing plates. A further object is to provide switches of the character above-stated in which the relatively stationary contact or contacts of a switch are slidably mounted in bushings and springpressed towards the center of the bimetallic disk, and in which stops limit the movement of the relatively stationary contacts in different manners according to the desired operating characteristics of the switch.

These and other objects and the advantages of the invention will be apparent from the following specification when taken with the accompanying drawings, in which:

Fig. 1 is a central vertical section through a switch embodying the invention; and

Fig. 2 is an elevation of the same.

In Figs. 1 and 2 of the drawings, the reference numerals 1 and 2 identify two plates which preferably are of metal, as shown, and with dished rims which meet at substantially the median plane of the lenticular casing which is formed by bending the outer edge 3 of the plate 1 over and around the rim of the plate 2. A narrow strip 4 is separated from the edge 3 and reversely bent to form a terminal lug for one branch of the control andalarm circuits, as will be described later.

The dished rim of the plate 2 terminates in a shallow radial flange 2 which spaces the adjacent rims apart and constitutes a seat for the edge of an imperforate domed bimetallic disk 5 which is designed and constructed in known manner to reverse curvature at the critical temperature at which the switch is to operate. The disk 5 rests loosely on its flange seat 2' and has two stable end positions at which its outer edge portion bears against one or the other of the annular seats formed by the inner surfaces of the dished rims of the casing plates 1 and 2. The bimetallic disk 5 constitutes a switch blade which moves between and in its alternate end positions engages one or the other of two relatively stationary contacts 7,

2,732,461 Patented Jan. 24, 1956 8 respectively which are mounted axially on and insulated from the casing plates 1 and 2 respectively.

The mountings for the contacts are identical and corresponding parts are identified by the same numerals but only the mounting for the contact 7 will be described. A metal bushing 9 extends through a central aperture of the plate l and is insulated therefrom by an intermediate washer 10 which fits within the plate aperture, and inner and outer washers 11, 12 of larger diameter. The enlarged inner end 13 of the bushing 9 is clamped against the inner insulation sheet 11 by an outer nut 14 which is slotted radially to receive the insulating covering 15 of a terminal wire 16 which is clamped against the nut 14 by a nut 17 having an inturned flange which seats upon the nut 14 and compresses the insulation 15 to provide a sealed chamber for the terminal connection.

The bushing 9 is counterbored from the outer end to receive the contact 7 which has a flange 18 constituting a stop for limiting inward movement of the contact by a spring 19 seated between the flange and a disk or washer 29 which is retained in the bore by spinning over the outer end 21 of the bushing. For convenience of description, the terminal wire for the contact 8, and its insulating cover will be designated by primed numerals 16 and 15 respectively.

The method of operation of the illustrated embodiment of the invention will be apparent from a typical control and alarm circuit which is shown schematically in Fig. 1. A current source which is shown schematically as a battery 22 is connected to the casing terminal 4 by a lead 23, a visual and/ or audible alarm 24 is connected to contact 7 by the wire 16, the controlled apparatus 25 is connected to the contact 8 by the wire 16, and the return circuits to the battery are completed by leads 26, 26 respectively. A switch 27 will be included in the lead 23 when the controlled apparatus 25 is energized by the source 22, but may not be necessary when the controlled element is a relay for regulating some larger apparatus in accordance with the ambient temperature at the bimetallic switch.

Under normal temperature conditions, the bimetallic disk 5 is domed toward the left, as shown, and in engagement with contact 8, thereby energizing the controlled apparatus 25 when the switch 27 is closed. Assuming that the apparatus 25 is to be tie-energized upon a rise .in ambient temperature to a preselected value, the concavity of the bimetallic disk 5 will decrease progressively with the temperature rise and it will eventually be planar and in unstable equilibrium at the critical value of the ambient temperature. The apparatus 25 remains energized during this period since the contact 8 is moved axially by the spring 19 to maintain engagement with the disk 5. The bimetallic disk engages the contact 7 before reaching its unstable midposition, and the alarm 24 is thus energized to warn the operator or attendant that the ambient temperature is approaching the critical value at which the apparatus 25 will be de-energized by a reversal of the curvature of the disk 5. If corrective action is not taken or is not effective, a further temperature rise will reverse the curvature of the bimetallic disk 5 and it will snap over quickly to separate from the contact 8 and seat against the rim of the plate 1 and the contact 7.

In many applications of the invention the ambient temperature varies at a relatively slow rate and a time lag between the ambient temperature and the temperature of the bimetallic disk 5 is of no significance. For use under conditions of rapidly varying ambient temperature, the casing plates 1, 2 and insulating sheets 11, E2 may be provided with openings 28 by which radiant heat may act directly upon the bimetallic disk 5, thus increasing the sensitivity of the thermal switch by reducing or eliminating a time lag of temperature response. The shape and the number of the openings 28 must of course be so selected that the strength of the casing is not adversely atfected.

Various modifications of the illustrated construction are possible within the scope of the invention. For example, the stop collars 18 of the contacts 7 and 8 may be so postioned that the bimetallic disk 5 separates from one contact before it engages the other contact, the cit contact 7 may be a fixed contact rigidly mounted on the plate 1, or the contact 7 may be omitted from switches for certain applications. In the latter case, the casing plate 1 will be a simple curved plate. Another modification comprises movable contacts mounted on the bimetallic disk and cooperating with contacts rigidly mounted upon the casing.

While the unstable neutral position of the disk usually will be its planar or mid-position, it is possible and practical to pre-stress the disk in such manner that the unstable neutral position is displaced to one side of the planar mid-position by as much as about 10% of the range of movement or deformation of the disk. With this construction, the disk will flex slowly with changing temperature over 40% of its movement from one end position, and then snap suddenly to its other end position as it passes through its unstable neutral position. The advantage of this asymmetrical characteristic is that it provides a relatively wide separation of the disk from the contact which it left in spite of the follow-up movement of that contact by its spring 19.

The invention is not limited to switches of any one size or range of sizes but is of particular utility in that it permits manufacture in small or so-called miniature sizes. The small size is of advantage for a number of reasons in addition to the initial advantage of low manufacturing costs. A small and light weight switch may be supported by the switch leads, and it responds to temperature changes at a point or small zone. When employed to shut down a motor or generator upon an abnormal temperature rise, the thermal switch of small size may be installed within the motor or generator casing at or adjacent a point of maximum temperature and thus eliminate the time lag which is characteristic of large size thermal switches which are positioned at the exterior of the motor or generator casing and which have an inherent time lag in view of their mass even when so dimensioned that they could be located within the casing of the apparatus to be protected.

The casing plates 1, 2 and the bushings 9 will be of metal for adequate strength and small thermal lag in the smaller size embodiments of the invention but they may be of a molded plastic material in larger size embodiments when small size and minimum time lag are not essential design requirements.

I claim:

1. A bimetallic snap-action switch comprising a lenticular casing formed of two sheet metal plates with dished rims meeting at substantially the median plane of the casing, an imperforate domed bimetallic disk constituting a movable switch blade, one of said metal plates having an outer edge extending over the other plate to hold the same in assembled position and said other plate having a shallow radial flange constituting a circumferential seat for said disk, the outer edge of said disk resting loosely on said seat and having two end positions according to the existing temperature at which its outer edge portion bears against the inner surface of the dished rim of one or the other of said casing plates, a contact mounted axially on one of said plates, and terminal means for said bimetallic disk and said contact; the terminal means for said bimetallic disk including said lenticular casing.

2. A bimetallic snap-action switch as recited in claim 1, in combination with a second contact mounted axially on the other of said casing plates and terminal means for said second contact.

3. A bimetallic snap-action switch as recited in claim 2, in combination with metallic bushings mounted on and insulated from the respective casing plates to support said contacts for axial movement, springs urging said contacts towards said bimetallic disk, and cooperating stop means on each bushing and its associated contact limiting movement of a contact to space the same from said disk in that end position thereof at which the concave disk face is towards that contact.

4. A bimetallic snap-action switch as recited in claim 3, wherein said stop means is so constructed and arranged that both contacts engage the disk at the unstable neutral position thereof.

5. A bimetallic snap-action switch as recited in claim 3, wherein said bimetallic disk is pre-stressed to displace its planar unstable neutral position to one side of its mid-position in said casing by up to l0% of its range of movement.

6. A bimetallic snap-action switch as recited in claim 1, wherein said bimetallic disk is pre-stressed to displace its unstable neutral position to one side of a plane through the edge of the disk.

7. A bimetallic snap-action switch as recited in claim 1, wherein a metallic bushing is mounted axially of and insulated from one of said plates, said contact is slidably mounted in said bushing, and a spring yieldingly urges said contact towards said disk.

8. A bimetallic snap-action switch as recited in claim 7, wherein said bushing is counterbored from the outer end thereof, said contact is cylindrical and has a stop collar thereon limiting inward movement of said contact by said spring.

9. A bimetallic snap-action switch as recited in claim 1, wherein one of said casing plates is provided with openings therethrough for radiant heating of said disk.

References Cited in the file of this patent UNITED STATES PATENTS 1,725,296 Paige Aug. 20, 1929 2,199,388 Bolesky May 7, 1940 2,207,601 Shakespeare et a1 July 9, 1940 2,239,540 Spencer Apr. 22, 1941 2,239,541 Spencer Apr. 22, 1941 2,477,801 Hathaway Aug. 2, 1949 2,514,807 Shaw July 11, 1950 2,596,171 Rabinow May 13, 1952 FOREIGN PATENTS 924,492 France Mar. 10, 1947 

